Cal. Pub. Util. Code § 15959

If two or more units of unincorporated territory are contained in the district, they shall be numbered, named, and designated by the board of supervisors in charge of the election of the first board of directors, and subsequently from time to time by the board of directors of the district, as unincorporated territorial unit number one, number two, number three, and so forth, there being as many numbers as there are units in the district.